1. Field of the Invention
The present invention relates to a packet forwarding apparatus, and more particularly, to a packet forwarding apparatus which is connected between a plurality of terminals and a session management apparatus and effectively forwards real-time packets.
2. Description of the Related Art
In recent years, as a voice communication technology, VoIP (Voice over IP) utilizing the Internet has been put to practical use. In VoIP, a virtual communication path (session) is established between the terminals prior to packet communication, and IP packets including voice data are transferred through the communication path. As session control protocols in IP multimedia communication, the IETF (Internet Engineering Task Force) specifies SIP (Session Initiation Protocol) RFC3261 suitable for VoIP and RTP (Real-time Transport Protocol) RFC1889 suitable for transmission of coded voice data.
SIP is a text-based application protocol utilizing a transport mechanism such as TCP (Transmission Control Protocol) and UDP (User Datagram Protocol), and a SIP message is comprised of a header for carrying request information or response information, and a message body for describing the contents of a session. For describing a session in the message body, for example, SDP (Session Description Protocol) RFC2327 is applied, and a correspondent terminal is identified by an SIP URI (Uniform Resource Identifier). As operation modes of an SIP server, a proxy mode and a redirect mode are known. In the proxy mode, the SIP server mediates a request for session establishment (call setup) between terminals. In the redirect mode, an originating terminal acquires information on a terminating terminal from the SIP server to directly communicate with the terminating terminal.
On the other hand, a VLAN (Virtual Local Area Network) is used in a enterprise local area network. The VLAN connects and groups together a plurality of terminals via a virtual LAN, independently of the physical connection configuration of the network. For example, in a network configuration in which a plurality of terminals are connected to an ISP (Internet Service Provider) network via an L2SW (Layer 2 Switch) which is a layer-2 packet forwarding apparatus and an L3SW (Layer 3 Switch) represented by a router, when a plurality of terminals connected to the L2SW form a plurality of VLANs, communication packets between VLAN segments are forwarded to the L3SW through the L2SW. Each communication packet is sent back from the L3SW to the L2SW after rewriting of the VLAN-ID by the L3SW, and forwarded to the connection line of a destination terminal.
In recent years, in order to ensure the confidentiality between terminals, VLAN aggregation (RFC3069) has been proposed. In the VLAN aggregation, a VLAN is assigned to each terminal connected to the L2SW and a unified VLAN is formed between the L2SW and the L3SW. Since a plurality of terminals connected to the same network have different VLAN-IDs in the VLAN aggregation, in the same way as with the above-described communication packet between VLAN segments, communication packets between terminals are forwarded to the L3SW through the L2SW, sent back from the L3SW to the L2SW after rewriting of the VLAN-ID by the L3SW, and forwarded to the connection line of a destination terminal.
As a related art with respect to sending back of packets by an L2SW accommodating a plurality of VLANs, Japanese Unexamined Patent Application Publication No. 2005-217715 proposes an L2SW that learns and stores the correspondence relationship between the IP address or MAC address of a terminal under control and a port number when an address resolution protocol ARP (or neighbor discovery protocol NDP) packet, which includes the address of the terminal as a destination address or a source address, or a response packet to the request was received. When a packet to be transferred between different segments is received, the L2SW directly forwards the received packet across the segments based on the learning result, without passing it through a higher-level router.
Further, Japanese Unexamined Patent Application Publication No. 2004-304371 proposes an L2SW connected to a plurality of hot standby routers (virtual routers) and a plurality of LAN segments. Upon receiving data to be transferred between first and second hosts belonging to different LAN segments, the L2SW converts the address of the received data by referring to a flow table and sends the received data to a destination host. In this case, the flow table stores the IP address of one of the first and second hosts as a source address, and the MAC address and IP address of the other host as a destination address. The L2SW learns the relationship between a source address and a destination address by passing the first received data through the virtual router, stores the learning result, and rewrites the destination MAC address of data received subsequently, using the MAC address indicated in a table entry corresponding to the source IP address of the received data.
In the case of implementing voice communication over an IP network such as the Internet, voice data is transferred in the form of an IP packet to the IP network. Since the IP network is shared by a number of users, network congestion retards the transfer of voice packets on the IP network. Particularly in an L2SW that accommodates a plurality of access lines forming VLANs or in an L2SW to which the VLAN aggregation is applied, voice delay is increased much more because of the travel between the L2SW and the L3SW if voice packets between different VLAN segments have to be sent back by the higher-level L3SW.
Since the L2SW disclosed in Japanese Unexamined Patent Application Publication No. 2005-217715 forwards all packets having address-resolved destination addresses to destination terminals without passing them to the higher-level router, the L3SW side cannot confirm actual traffic. Accordingly, in the case where the L2SW is applied to a network configuration in which a session management server connected to the L3SW side collects account information on voice communication for each session, there is a problem that the session management server cannot collect account information. Similarly, the L2SW disclosed in Japanese Unexamined Patent Application Publication No. 2004-304371 forwards all data except the first received data to destination terminals without passing them to the higher-level router. Therefore, there is the same problem as in Japanese Unexamined Patent Application Publication No. 2005-217715.